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Most of us compare the world today with what we remember from our childhoods, but no further back. If changes happen slower than human generations, we tend not to notice unless we have written records.

This is a story of a secret hidden in one of these records, unearthed after nearly 70 years.

Of the five species of native fishes whose babies we catch and eat as whitebait, the banded kokopu (Galaxias fasciatus) is relatively common - which means it's the only one not yet under threat of extinction.

Curiously, banded kokopu are almost absent from the Whanganui area. The only healthy population we know about between Taranaki and the Manawatū were discovered just a few years ago living in a drainage ditch behind Karaka Street in Castlecliff. This could be a natural gap in their distribution - perhaps they were never here - but it's telling that banded kokopu prefer to live in low-lying wetlands, and wetlands in

Whanganui/Manawatū were almost all drained long ago and converted to farmland.In a collection store at the Whanganui Regional Museum is a cabinet of specimens preserved in alcohol, some over 100 years old. Just after World War II, the museum's curator was Jock Moreland, an enthusiastic natural historian who went on to be the Curator of Fishes at the Dominion Museum for many years.

Fish expert Stella McQueen was recently going through our museum's archived correspondence, and discovered that in 1948 someone gave Moreland a large fish they'd caught at Kaitoke. It looked different from the fishes he was familiar with, so Moreland sent it to fish expert Gerald Stokell at Canterbury Museum, who identified it as a giant kokopu (Galaxias argenteus), an unusual species for Whanganui, and certainly not one you'd find at Kaitoke today.

What Moreland told Stokell was interesting. He pointed out how unusual it was to find a yellow-spotted native fish. "... the many I have caught in the past have all been of the brown barred variety, which are quite common here."

Moreland is referring to banded kokopu.

So in the 1940s the banded kokopu was the most common native freshwater fish in the Whanganui area.

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By the time regular scientific surveying was happening in the 1980s, they had almost vanished.

Because our native fishes are nocturnal, people only tend to notice them in whitebait season. If it wasn't for Jock Moreland's short typewritten letter, preserved as a carbon copy in the museum archives, we would have no idea that banded kokopu were ever present in the Whanganui area, let alone abundant, and that they'd quietly disappeared over just a few decades.

That a native species which is not even supposed to be threatened could vanish from an entire region without anybody noticing should make us think. Our whitebait species are under pressure throughout the country from commercial fishing, farming, and declining water quality, and yet we have very little idea how abundant and widespread they used to be, because our memories and survey data don't stretch back that far.

Since the 1980s, researchers have been surveying New Zealand waterways and noting down the fish species present in a database. To identify historical changes before then we need to look for scraps of information in archives and museums, and it's the job of natural history museums to continually collect and preserve a record of our biodiversity.

This can take many forms: a skull, a stuffed skin, a pickled fish in a jar, or just a short letter to a colleague.

*Mike Dickison is curator of Natural History at the Whanganui Regional Museum.Image captionThis banded kokopu was caught near Brunswick in 1898 and kept in the Museum aquarium by founder Samuel Drew. Later he preserved it and put it on display.Whanganui Regional Museum 1800.422